The immunogenic and metastatic properties of the BL6 and 3LL tumor cells treated with MNNG or UV light were further investigated. Tum- clones from UV-treated 3LL cells have complete cross reactivity with nonhomologous tum-, tum+ and parental 3LL tumor cells. Parental 3LL tumor cells were sensitive targets for specific cytotoxic T cells, and they were equally efficient as immunogenic tum- cells in the cold target inhibition assay, but they were poorly immunogenic in in vivo and in vitro tests. These results indicate that parental 3LL cells possess tumor associated antigenic determinants but express them in nonimmunogenic form. The primary antitumor response was highly sensitive to the immunosuppressive action of x-irradiation (550R) or cyclophosphamide (Cy) (200 mg/kg) treatment. However, antitumor resistance of the preimmunized mice remained fully expressed after these treatments. Cy treatment decreased the total number of spleen cells and the proportion of B-lymphocytes, but substantially increased the proportion of Ly 2.2+ and L3T4 lymphocytes. Ly 1.1+ lymphocytes were mostly responsible for the adoptive transfer of anti-3LL resistance. MNNG treatment of BL6 melanoma cells increased the expression of class I MHC antigens and immunogenicity of the treated cells. Mice which rejected a tum- clone from the BL6T2 subline were resistant to the nonhomologous tum- melanoma clones, and to tum+ clones which expressed high level of H-2b antigens, whereas tum+ clones with low level expression of the H-2b complex or parental BL6 melanoma with little or no detectable MHC antigens, were able to grow in immune mice. Interferon treatment increased the expression of class I MHC antigens; however, it did not influence the ability of BL6 melanoma cells to grow in vivo, perhaps due to transient effect of interferon on the H-2b antigen expression. The metastatic properties of MNNG-treated BL6 melanoma cells was substantially reduced, probably due to the increase of their immunogenicity. Further understanding the mechanisms responsible for the conversion of the nonimmunogenic tumor cells into the highly immunogenic variants could open the way for the utilization of specific immune mechanisms for the immunotherapy of cancer patients.